Add _name method to Trigger base class

[mkusaka]

Summary

Added a _name static method to the Trigger base class. Trigger.encode calls self._name() but the base class did not define it, causing AttributeError on unrecognized trigger types.

Background

• All Trigger subclasses define @staticmethod _name() returning their class name, but the base class did not
• decode_trigger() returns a bare Trigger instance for unrecognized trigger types (line 79), and calling .encode on it crashed

Example

from iterm2.triggers import decode_trigger

# Unrecognized trigger type returns a base Trigger instance
encoded = {
    "action": "SomeFutureTrigger",
    "regex": "test",
}
trigger = decode_trigger(encoded)
trigger.encode  # AttributeError: 'Trigger' object has no attribute '_name'